Blog for the RepRap project at www.reprap.org - a project to create an open-source self-copying 3D printer. To get all the early posts on this blog with all the images as a single PDF visit this page.

Saturday, April 28, 2007

Aluminum Barrel Success!

After letting the JBWeld cure, I was ready to get down to business and try some extrusion tests. I had my nozzles arrayed before me, my power supply ready to rock, and some alligator clips ready to clip. I knew the 0.7mm and bigger ones would extrude, so what I was really interested in was how the 0.4mm and smaller ones would perform. I had my doubts... but they were completely wrong. I usually try to save the best for last, but I cant. All nozzles (including the 0.25mm nozzle) extruded beautifully and at a good rate. I took measurements with my cheapo digital calipers. They may be a bit off, but they were close enough.

those last two were very surprising for me. i honestly didnt think the 0.25 one was going to work at all. boy was i wrong. they all extruded very well. the smaller ones werent even very slow. the HDPE came out fine, and in a nice even stream.

next, i wanted to try some of the more advanced polymers that Sebastien sent me. first up was ABS. i cleared a barrel, loaded up a length, and started to heat it up. i waited a few minutes and tried some extrusions. it just wasnt having it. i may have been impatient and gave up too early, because when i pulled the ABS out to examine it, it was definitely malleable and soft. i dont know if it goes transparent like HDPE does when its melted, but it looked like it was nearly there. perhaps i need a hotter barrel, or to wait longer. i'll find out soon =)

after that, i decided to try the UHMW HDPE (this is basically a *very* nice version of HDPE). i loaded some up in a fresh barrel, turned it on, and waited a few minutes. this stuff extrudes like a DREAM! it came out faster, and better than regular HDPE. i dont have any hard numbers to back it up, but I have a feeling that this stuff could become my polymer of choice (until i can get ABS to work.)

i dont have any pics for this one as it is really hard to photograph the filament. anyone know where i can get reverse graph paper? (black paper, white lines?) i suppose i could just use normal black paper, but i like the way the graph paper gives you scale automatically. picked that habit up from Adrian =)

ABS, at least the white pigmented ABS that I've used, doesn't go transparent. I don't know what virgin ABS would do. What I found is that it requires not so much a very hot extruder barrel, though that helps, as a lot more pressure from the polymer pump.

That's why I'm redesigning the Mk 2 so that it can deliver more pressure. The only problem that I can see is that HDPE puts me right at the top level of torque that the GM3 can deliver before the clutch kicks in already with HDPE. We have have to find a higher torque gearmotor for an ABS extruder.

You may want to go to 2 amps for ABS. You can do HDPE on 1 but it's a bother. Just half the length of your nichrome and you've got it.

the thread is standard american 3/8". the aluminum bolts and such will need to be ordered from a specialty place. although i plan on making up barrels and nozzles for anyone that ones them. using an equivalent metric size would work fine too.

i'm trying the ABS as we speak. hopefully i can get it to work. i'm running at 1.3 amps. i'll make a 2 amp barrel this week to do further tests. i just dont think its getting hot enough. i also want to try a slightly different approach to wire wrapping / heater barrel fabrication.

as for the drill bits... i got them from http://drillcity.stores.yahoo.net/index.html this place really rocks.

Zach, special ordering the bolts may not be needed, most big box hardware stores sell 3' lengths of aluminum bar stock that is not too difficult to machine into shape . I made a few prototype heads a couple of days ago that I haven't yet had a chance to test.

Sorry if I'm teaching people things that they know, but Zach had a bit of trouble breaking drill bits, especially the fine ones. When drilling deep, or fine, use a woodpecker cycle; that is drill a short way, then raise the drill bit to clear the swarf and brush it off if need be, then drill a little more and so on. It's slower, but it stops the swarf fouling the bit and causing it to snap.

This is a particular problem with aluminium, as its so soft, which you'd think would make it easy to machine, but paradoxically doesn't.

ABS has some insoluble polybutediene (the B in the acronym) that should make it translucent even when melted. This means microscopic gobs of rubber that blunt a passing crack tip, making the material shatter-resistant. I have seen transparent ABS in the case of a TI calculator, but I think they probably had some kind of a copolymer in the butyl rubber to match its index of refraction to the PS/acrylic matrix.

By the way, it's these little gobs (micelles, technically...) of rubber that differentiate ABS and "high-impact" plastic from the ordinary polystyrene used in model kits and picnicware. The "A" is acrylonitrile, added for strength.